The Dirty Paper Coding (DPC) principle proposed early in 1983 points out, when the transmitter side has already known the interference signal, the channel capacity is not influenced by the existence of interference signal. DPC is paid attention and thought as good study model of a variety of communication problems until it is associated with a plurality of communication problems. Its primary application is focused on digital watermarking, precoding design for eliminating interference and a variety of broadcast strategies, etc. In recent years, DPC is widely used in the field of multi-user communication.
Tomlinson-Harashima Precoding (THP) was firstly designed to eliminate inter-symbol interference (Ref 1). Under the circumstance of existence of interference, the transmitter side can transmit the intended signal to the receiver side by the process of pre-subtraction of the interference. The solution of pre-subtraction of interference is similar to pre-subtraction of multi-user interference. Therefore, THP can be easily extended to eliminate multi-user interference. After the year 2000, THP is associated with DPC and is interpreted as one-dimensional implementation of DPC (Ref 2).
THP is attractive due to its simple implementation. FIG. 1 illustrates the principle diagram of THP method with interference and additive Gaussian noise. With this technology, intended signal v of the dirty paper precoding pre-subtracts interference signal s at the transmitter side and obtains difference signal d. Then, the d modulo-M (an integer) signal is obtained via the quantization process by quantizer Q. what the transmitter sends is output signal of Q, also d modulo-M (an integer) instead of the difference signal d to make sure the output signal within the power constraint. At the receiver side, intended signal can be reconstructed as {circumflex over (v)}, that is, the estimate signal of the intended signal after the receiving signal y is quantized by the same quantizer Q.
However the prior arts (Ref 1, 2) only discuss and solve the problem from the amplitude point of view. The inventor of the application thinks that the modulation phase should also be considered to reconstruct v from the received signal y optimally. Especially in case that interference signal s has larger amplitude than that of intended signal v, it is quite difficult for the prior arts to reconstruct the phase of intended signal. FIG. 2 and FIG. 3 show the problem of prior art in case of BPSK and QPSK respectively. For simplicity, assuming that v and s have the same modulation constellation. The interference signal s has higher amplitude than intended signal v. In FIG. 2 different constellation points of v are distinguished by different shapes while different constellation points of s are distinguished by different shadow fillings. Operation (v-s) in FIG. 1 results in the constellation points as shown in FIG. 2(c). Each constellation point shown in FIG. 2(c) has both information (comprising amplitude and phase information) of intended signal v and the interference signal s. Then modulo operation of the difference signal (v-s) will decrease the distance between different constellation points in the same quadrant. Thus, the possible constellation points of the received signal y are shown in FIG. 2d. It can be seen that the constellation of the received signal y is the enlarged constellation of signal v from QPSK to 16QAM. Furthermore, for the received signal y, different constellation points with same shapes and different shadow fillings are isolated at different quadrants. Obviously, phase reconstruction of intended signal v from the received signal y can hardly achieve. The case of QPSK is shown in FIG. 3, which has the similar problem to FIG. 2.
Ref 3 is an improvement of THP method described in Refs 1 and 2, which is named as SDPC (Structural Dirty Paper Coding). This method discusses the improvement of THP method on the assumption that the receiver side has known the modulation structure of the interference signal. FIG. 4 illustrates the principle structure of SDPC, where the source signal u is performed by quadrature amplitude modulation. By precoding the modulated signal v, since the receiver side has known the modulation structure of the interference signal and the receiver has also prior knowledge of the constellation of the received signal y; therefore, SDPC has better demodulation performance than THP. However, the phase reconstruction problem is solved to some extend at the cost that the receiver knows the modulation structure of the interference signal. This problem is shown in FIG. 5. What's more, in case that source signal and interference signal are both QPSK, the receiver side has to perform 16QAM decoding on the received signal y whose constellation is enlarged to reconstruct the phase of source signal.
Ref 1, 2, 3 are listed as below.
Ref 1: H. Harashima and H. Miyakawa, “Matched Transmition technique for channels with intersymbol interference”, IEEE Trans. Commun., vol 20, pp. 774-780, August 1972
Ref 2: W. Yu and J. M. Cioffi, “Trellis precoding for the broadcast chamiel”, in Proc. IEEE Global Telecommunications Cont. GLOBLECOM'2001)
Ref 3: Bin Liu, Hui Liu, Sumit Roy, “Structured dirty paper coding with known Interference structure at receiver” IEEE Signal Systems and Computers, 2005